Problem: In physics class, the girl to boy ratio is $5$ to $7$. If there are a total of $60$ students, how many girls are there?
Explanation: A ratio of $5$ girls to $7$ boys means that a set of $12$ students will have $5$ girls and $7$ boys. A class of $60$ students has $5$ sets of $12$ students. Because we know that there are $5$ girls in each set of $12$ students, the class must have $5$ groups of $5$ girls each. There is a total of $25$ girls in physics class.